Colorado Public Meeting Republican River Compact Administration Groundwater Model and RRCA Accounting Colorado Public Meeting Michael Sullivan CDWR May 16, 2016

Compact Basics Virgin Water Supply (VWS) Allocation for Each State Annual Accounting Required

Accounting Compact allows each state an amount of depletion of the Virgin Water Supply (VWS) Surface water use, tributary GW use, and Reservoir evaporation in pre-FSS accounting SM found all GW impacts had to be accounted (i.e. Ogallala Aquifer Use) Updated the accounting for all GW use Now need to calculate each State’s Calculated Beneficial Consumptive Use (CBCU)

Accounting Formula Sub-basin VWS = Gage +All CBCU + ΔS – IWS Main-stem VWS = Hardy Gage – Σ Sub-basin gages + All CBCU in the Main Stem + ΔS – IWS CWS = VWS – ΔS – FF Allocation for state in Sub-basin and Main Stem = CWS x % States Allocation = Σ Allocation for each state States CBCU = Σ State’s CBCU in each Sub-basin and Main Stem These last two are inputs to Table 3 of the accounting which determines compact compliance

Example Arikaree River   CBCU Colorado = 0.6 x Dc + % x Pc + 0.5 x M&Ic + EvNFRc + GWc CBCU Kansas = 0.6 x Dk + % x Pk + 0.5 x M&Ik + EvNFRk + GWk CBCU Nebraska = 0.6 x Dn + % x Pn + 0.5 x M&In + EvNFRn + GWn VWS = Arikaree Gage at Haigler Stn. No. 06821500 + CBCUc + CBCUk + CBCUn – IWS CWS = VWS - FF Allocation Colorado = 0.785 x CWS Allocation Kansas = 0.051 x CWS Allocation Nebraska = 0.168 x CWS Unallocated =-0.004 x CWS

Accounting (continued) Accounting includes: SW Diversions Pumped Diversions Reservoir evaporation Federal Reservoirs Non-federal Reservoirs GW CBCU Irrigation use M&I use Imported Water (IWC) Augmentation Streamflows Flood flows

General Need for a Model Need to know how much GW use impacted the VWS but we can’t ‘see’ groundwater Must calculate it’s flow and impact Model simply uses physics to calculate groundwater flow Geology gives us flow parameters Topography gives us flow directions and intercepts Gravity and withdrawals influence flow Climate, cropping, consumption influence returns

Republican Requirements Develop and calibrate a groundwater model of the Republican River Basin in order to calculate the impact of groundwater pumping and surface water imports on compact streams Apply the groundwater model annually to evaluate pumping and surface water imports for annual compact accounting

Republican Application Run model with all three states pumping on Run model with Colorado off and NE/KS on Run model with Kansas off and CO/NE on Run model with Nebraska off and CO/KS on Run model for Nebraska Mound Credit Result How much CBCUgw for each state

Origin of the Model USGS Lincoln Field Office Work started 1997 Open File Report June 2002 Groundwater Technical Committee Agreement to cooperate Spring 2002 Work started July 2002 Final Settlement Stipulation Dec 2002 Model submitted to Special Master June 30, 2003

Conceptual Model Single aquifer to Pierre Shale Surface water basin extended to Platte River Major man-made stresses recharge and wells Well Pumping Irrigation returns Canal leakage Major natural stresses precipitation, ET and streams Precipitation recharge major inflow Only baseflow simulated Minor stresses springs and underflow

Model Calibration Head at 10835 wells Baseflow at 56 gages Selected wells given greater weight Trends in water levels important Baseflow at 56 gages Base flow estimated by pilot point method Model stream flow is only base flow Impact defined as change in base flow

Model Application Groundwater Computed Beneficial Consumptive Use (CBCU) evaluated by switching off all wells and associated recharge within the model domain in a state Imported Water Supply (IWS) evaluated by turning off all surface water returns and canal leakage in mound area CBCU & IWS are the difference between “historical” and “impact” run stream flow and reservoir leakage.

Republican River Basin Topographic

Model Domain Note that the model domain is based on the geology of the Ogallala Aquifer It is not based on the topographic divide of the Republican River Basin The Model Domain Includes Areas in CO, KS, and NE Exceed the Topographic Basin.

Summary FSS requires GW CBCU be included in the accounting as use of a states allocation A GW model is the method adopted by the FSS and US Supreme Court to calculate GW CBCU GW CBCU is included in the RRCA accounting Accounting is Performed Annually